The Democrats stepping into the fray so far include three veteran state lawmakers: state Sen. Linda Greenstein (D-Middlesex), and Assemblymen Upendra Chivukula (D-Somerset) and Reed Gusciora (D-Mercer). Mercer County Executive Brian Hughes could also make a run for it. Holt's district — which includes includes Princeton, Trenton and suburban towns in Middlesex County — leans Democratic.

On the Republican side, Alieta Eck, who ran unsuccessfully for the U.S. Senate last year, has already declared her intentions to run for Holt's seat. Other possible GOP candidates include Scott Sipprelle, a venture capitalist who challenged Holt in 2010, and John Crowley, a biotech executive who has been mentioned frequently as a congressional contender.

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